Improve config of LESS directories in the Phakefile

If we ever need to change where LESSPHP will look for additional include files, we have to change something in the middle of the code. Please, there should be a way to move that into the constants file (by using a variable like $LESSDIRS, although I tried quickly and it didn't work), or another way. The last option would be creating this config in the top of the Phakefile.